In recent years, there have been proposed and developed various pulley thrust control technologies for belt-drive continuously variable transmissions (CVTs), which enable an actual transmission ratio to be steplessly adjusted or feedback-controlled toward a desired transmission ratio. As is generally known, a belt-drive CVT uses a drive belt (usually, a single segmented steel belt) running in primary and secondary variable-width pulleys whose V grooves are aligned with each other, to provide varying gear ratios or pulley ratios. In more detail, the V groove of the primary pulley, to which input rotation is transmitted from an engine, is constructed by a stationary flange and an adjustable flange axially slidable for varying the width of the V groove of the primary pulley by way of a primary pulley thrust resulting from a “primary pulley pressure”. The V groove of the secondary pulley, which is connected via a gear train to drive wheels, is constructed by a stationary flange and an adjustable flange for varying the width of the V groove of the secondary pulley by way of a secondary pulley thrust resulting from a “secondary pulley pressure”. Actually, the adjustable flange of the primary pulley is forced toward the associated stationary flange by the primary pulley pressure, and simultaneously the adjustable flange of the secondary pulley is forced toward the associated stationary flange by the secondary pulley pressure. This enables power transmission between the primary and secondary pulleys via the drive belt, while keeping the drive belt in friction-contact with the V grooves of the primary and secondary pulleys. During the power transmission, assuming that the frictional force between the variable-width pulley and the drive belt is less than a belt driving force, a belt slippage occurs. This deteriorates the durability of the belt-drive CVT. To avoid such a belt slippage, a lower limit (or a lower limit guard) of the pulley thrust of each of the primary and secondary pulleys is set. However, when the pulley thrust of either one of the primary and secondary pulleys must be reduced during a shift, the lower limit guard undesirably acts to disturb an adequate drop in pulley thrust. This deteriorates a gear shift responsiveness. To avoid this, the pulley thrust of the other pulley is increasingly compensated for to ensure the permissible gear shift responsiveness. Increasing the pulley thrust of the other pulley means an increase in engine load, thus deteriorating fuel economy. To balance contradictory requirements, that is, a reduced fuel consumption (improved fuel economy) and a permissible gear shift responsiveness, in particular, an adequate transient gear shift responsiveness, Japanese Patent Provisional Publication No. 2001-173770 (hereinafter is referred to as “JP2001-173770”) teaches (i) setting a lower limit guard for each of pulley thrusts of primary and secondary pulleys, (ii) calculating a pulley thrust of a lower-thrust pulley of the two pulleys so that a radius of rotation of the drive belt of the lower-thrust pulley side, in other words, half of the effective diameter of the lower-thrust pulley, is brought closer to a radius corresponding to a desired transmission ratio or a desired pulley ratio, and (iii) holding the calculated pulley thrust of the lower-thrust pulley at the lower limit guard when the calculated pulley thrust of the lower-thrust pulley becomes below the lower limit guard. JP2001-173770 also teaches (iv) calculating a difference between the calculated pulley thrust and the lower limit guard and (v) adding the calculated difference to a pulley-thrust command value of the higher-thrust pulley, in order to avoid the transient shifting responsiveness from deteriorating owing to a decrease in the time rate of change in the radius of rotation of the drive belt of the lower-thrust pulley, arising from holding the calculated pulley thrust at the lower limit guard of the lower-thrust pulley (see the difference and the additional value shown in FIGS. 5A and 7A). This balances two contradictory requirements, that is, the drive-belt slippage prevention and better shifting response.